Playback Rate Suddenly Changes
There is a pretty good explanation from Mikeycal Meyers.

It's about the "Refresh Sequencer" function and the MPEG Preseek option (set to its maximum of 50) in each video Strip Property. If this won't help you, try to use Proxies which take some time to be built, but they should not only improve your cutting workflow but eliminate your issue as well.